Every organism needs organic molecules to feed into respiration. There are only two basic ways to get them: build them yourself, or take them from something else. Everything on this page — plants, fungi, euglena, archaea living on hydrogen gas — is a variation on those two.
📘 What you need to know
Your mode of nutrition is the way you gain organic molecules to fuel respiration.
Autotrophs make their own organic molecules from simple inorganic substances. Heterotrophs take them from other organisms.
Photoautotrophs use light energy; chemoautotrophs use energy from oxidising chemicals.
Saprotrophs secrete enzymes onto dead material and digest it externally, then absorb the products.
Mixotrophs use more than one mode. Obligate mixotrophs need both; facultative ones top one up with the other.
Archaea vary: phototrophic, chemotrophic (chemoautotroph or chemoheterotroph) and heterotrophic.
Why nutrition exists at all
Organisms need energy in the form of ATP. The energy stored in ATP comes from organic molecules such as carbohydrates, and it is transferred during respiration.
So every organism has to get hold of organic molecules from somewhere. The way it does that is its method, or mode, of nutrition.
Watch your language here — examiners are strict about it. Energy is never produced or created, only transferred from one form to another. Photoautotrophs do not produce energy; they produce their own food by transferring light energy into chemical energy.
Keep this shape in your head. Almost every nutrition question is asking you to place an organism somewhere on this diagram.
Autotrophs and photosynthesis
An autotroph synthesises its own organic molecules from simple inorganic substances in its environment.
Photoautotrophs use light energy.
Chemoautotrophs use energy from the oxidation of chemicals.
Photosynthetic organisms are photoautotrophs. They use light energy to convert carbon dioxide from the air into organic molecules such as carbohydrates, using photosynthetic pigments such as chlorophyll to absorb the light.
Photosynthesis
carbon dioxide + water → glucose + oxygen • requires light and chlorophyll
Because they make their own organic molecules without relying on anything else, photosynthetic organisms are called producers. Two things follow, and both matter:
Photosynthesis transfers light energy into a chemical form that living organisms can use. Producers are then eaten, and the energy transfer continues along the food chain.
Photosynthesis is also responsible for releasing oxygen into Earth’s atmosphere, which is what makes aerobic respiration possible in the first place.
Photosynthetic organisms include plants (terrestrial and aquatic), algae (single-celled algae and multicellular seaweeds) and photosynthetic bacteria such as cyanobacteria.
Heterotrophs: holozoic nutrition
A heterotroph gains organic molecules from the tissues of other organisms. All animals are heterotrophs.
Organisms using holozoic nutrition do it in four steps:
🧩 The four stages of holozoic nutrition
Ingestion — eating.
Digestion — breaking larger molecules into smaller ones.
Absorption — transporting those molecules from the digestive tract into the cells.
Assimilation — using the molecules to build cells and tissues.
The one thing to remember: holozoic nutrition involves internal digestion. That is what separates it from the saprotrophs below.
There are heterotrophs that break this rule. House flies, for example, secrete enzymes onto their food and then absorb the products. They are heterotrophs, but they are not holozoic, because the digestion happens outside the body.
Saprotrophs
Saprotrophs are heterotrophs that feed on dead organisms and waste material by secreting enzymes onto their food and digesting it externally, then absorbing the products. Examples are fungi and bacteria, which is why we also call them decomposers.
An earthworm and a fungus both live on dead material, but only the fungus is a saprotroph. The worm is a detritivore using holozoic nutrition.
Why saprotrophs matter to everything else
They secrete a wide range of digestive enzymes, which lets them hydrolyse a large variety of biological molecules and release a large range of products.
Those products include mineral ions such as ammonium ions and phosphate ions.
Crucially, not all of the products get absorbed by the saprotroph. Some minerals are left in the surrounding soil, where other organisms — plants especially — can absorb them.
Without saprotrophs, the nutrients locked up in dead and waste matter would never be released, and plants would not have enough minerals.
Decomposers and detritivores are not the same thing. Both feed on dead material, but decomposers such as fungi are saprotrophs using external digestion, while detritivores such as earthworms digest their food internally, using holozoic nutrition. Examiners test this exact distinction.
Mixotrophs
Some organisms use more than one method of nutrition, such as autotrophy and heterotrophy together. These are mixotrophs.
Obligate mixotrophs must constantly have access to both methods.
Facultative mixotrophs can survive on one method, which is supplemented by the other.
The standard example is euglena, a single-celled eukaryote found in fresh water:
It has chloroplasts and photosynthesises, and a light-sensitive spot that lets it position itself so the most light reaches those chloroplasts.
It also takes in bacterial cells by endocytosis and digests them using enzymes stored in lysosomes.
Other mixotrophs worth knowing:
Carnivorous plants — they build organic molecules by photosynthesis and from the tissues of digested insects.
Corals — the polyps gain organic molecules from their symbiotic photosynthetic algae and by filter feeding.
Marine plankton such as dinoflagellates — many feed on bacteria while also photosynthesising.
Nutrition in archaea
The archaea are a diverse group of single-celled organisms making up one of the three domains, and they vary a lot in how they feed.
Group
Where the ATP energy comes from
Where the carbon compounds come from
Phototrophic archaea
Light. Some use a pigment called bacteriorhodopsin to absorb light and pump hydrogen ions across a membrane; the resulting ion gradient drives ATP synthase.
From other organisms — so these are photoheterotrophs, not autotrophs.
Chemoautotrophic archaea
Energy released from chemicals, transferred to carbon compounds by chemosynthesis. Sources include hydrogen gas, ammonia, methane and hydrogen sulfide.
They make their own, from inorganic sources.
Chemoheterotrophic archaea
Energy from chemicals, used to drive ATP production directly.
From other organisms.
Heterotrophic archaea
From the carbon compounds they take in.
From other organisms, e.g. breaking down and absorbing compounds in dead plant material.
Note the split. “Photo-” and “chemo-” tell you the energy source. “-autotroph” and “-heterotroph” tell you the carbon source. They are two separate questions, which is why photoheterotrophs can exist.
You are not expected to give examples of archaea at species level. Knowing the four groups and what makes them different is enough.
Worked examples
WORKED EXAMPLE
Distinguish between holozoic and saprotrophic nutrition. [3]
Start with what they share
Both are forms of heterotrophic nutrition.
Now the difference that mattersHolozoic: food is ingested first and digested inside the body.Saprotrophic: enzymes are secreted onto the food and digestion happens outside the body.Saprotrophs then absorb the products of that external digestionthe word “internal” or “external” has to appear. It is the deciding point.
WORKED EXAMPLE
Explain why saprotrophs are essential to a forest ecosystem. [3]
Say what they do to dead matterThey secrete a wide range of enzymes that hydrolyse the molecules in dead organisms and waste.What is released?Mineral ions such as ammonium and phosphate are released into the soil.Who benefits?Not all the products are absorbed by the saprotroph, so plants can take up the restwithout this step, nutrients stay locked in dead matter forever.
WORKED EXAMPLE
A single-celled organism has chloroplasts and also engulfs bacteria. State its mode of nutrition and justify your answer. [2]
Identify both methods being usedChloroplasts mean autotrophy; engulfing bacteria means heterotrophy.Name the term for using bothIt is a mixotroph, because it uses more than one method of nutritioneuglena is the example to name if the question invites one.
💡 Exam tip
Never write that energy is produced or made. Energy is transferred.
Learn the four holozoic stages in order and be able to define each in a few words.
For saprotrophs, the key phrase is “secrete enzymes and digest externally”.
Split the prefixes: photo/chemo = energy source, auto/hetero = carbon source.
If you name euglena, mention both its chloroplasts and its endocytosis. One alone does not show mixotrophy.
Mention that photosynthesis put the oxygen in the atmosphere if a question asks why it matters globally.
⚠ Common mix-up
Calling decomposers and detritivores the same thing. Decomposers digest externally; detritivores digest internally.
Assuming every heterotroph is holozoic. House flies and fungi are heterotrophs that digest outside.
Thinking autotroph means “plant”. Algae, cyanobacteria and chemoautotrophic archaea are all autotrophs.
Confusing digestion and absorption. Digestion breaks molecules down; absorption moves them into cells.
Confusing absorption and assimilation. Assimilation is using the molecules to build cells and tissues.
Assuming phototrophic archaea are autotrophs. Halobacteria use light for ATP but still take carbon compounds from other organisms.
Saying producers “create energy”. They transfer light energy into chemical energy.
Up next: Nutrition in Hominidae — how you can read an animal’s diet off its teeth, and why that trick is less reliable than it looks.
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